1. Field of Invention
The present invention relates to vehicle aerial devices and more particularly to an improved mechanism which permits the articulation of an upper boom relative to a lower boom and utilizing two cylinders disposed in a vertical plane and connected to a respective one of the booms.
2. Description of Prior Art
Vehicle aerial devices usually consist of an upper boom equipped with a basket to carry a workman, a lower boom and a pedestal which is mounted on the bed of a truck. The upper and the lower booms could be both articulated by cylinders via a hydraulic control system that can also rotate the lower boom. They are used for a variety of applications where it is necessary to access locations remote from the ground, like the servicing of overhead power lines, for example. In this field of work, one of the major concern relates to the fact that workmen should be insulated from the ground as it is extremely hazardous to work close to transmission lines.
In order to protect workmen from electrocution, manufacturers of such devices provide an upper boom, a lower boom, or both booms made from insulated materials, the most common being fiberglass reinforced plastics.
The more insulation material used on the aerial device, the safer it is for the workman, as he is close to the transmission lines. In other words, a fiberglass boom which is longer means better insulation against electrocution. Most aerial devices use the four bar or the "scissors linkage" mechanism with a hydraulic actuator to raise or lower the boom. It is made of five parts; a cylinder, a lower boom arm, an upper boom arm and two different connecting links. Although this mechanism is relatively easy to maintain and capable of handling large loads for most of the operating range, the cylinder is usually quite long which makes the lower boom arm difficult to insulate. Another disadvantage of the mechanism relates to the forces acting on the components, especially on one of the connecting links, which vary rather widely as the angle of articulation changes and as a constant moment is applied.
Constant radius mechanisms using sprockets and chains or pulley and cables have also been employed to rotate the upper boom relative to the lower boom. Some advantages of this mechanism include large angle of rotation of about 270.degree., constant moment applied to the pulley or sprocket and constant angular velocity over the range of the articulation. However, such systems requires cumbersome assemblies with many pieces at higher production cost. The metal surface of the articulated mechanism exposed to the power lines is great with the lower insulated boom insert tending to be quite small. Further disadvantages include looseness between the sprocket and chain, susceptibility to wear and frequent maintenance.